What is the ruling on a slave who divorces his wife twice and subsequently gains freedom?

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When a slave divorces his wife twice and is subsequently manumitted, the wife is unlawful for him unless she marries another husband and consummates that marriage. This is because the divorce constituted an irrevocable prohibition which can only be lifted by a subsequent marriage and consummation. This is the apparent position of the Madhhab. However, there is a narration from Ahmad stating that it is permissible for him to remarry her while retaining her under a single divorce count. This view is supported by the hadith of Ibn Abbas narrated from the Prophet, peace be upon him, regarding slaves: 'If he divorces her two pronouncements, and then they both become free, he may marry her.' Ahmad initially favored the first opinion but later expressed a preference for the tradition of Uthman and Zayd over the narration of Ibn Abbas, whose narrator 'Amr ibn Maghith he questioned. Abu Bakr stated that if the hadith of Ibn Abbas is authentic, action should be based upon it; otherwise, action should follow the hadith of Uthman and Zayd, which is the position followed.